First thing I'd check is to see if you have enough drive space available and rights to write to the specified output directory. Also, see if the output directory still exists. Other than that we'd need some more infos about where the file is going, where the sources are located, how large the project is, how much drive space is available, which OS and Flare version, and if it ever worked before.One more thing, you don't happen to have the old file open?

I found a solution: I am working in Russian language files and Flare didn't like the use of Russian under the Target folder > Basic Tab > Output File field. I changed the language from Russian back to English in the Output File field and now have no problems with publishing.